In the case of wireless communication, a reception signal is caused distortion due to influence of reflection, diffraction, dispersion or the like which is caused by an obstacle such as a building or the like. Therefore, a general reception device is requested to realize reliable reception performance by estimating a transmission line from the reception signal and correcting the distortion of the reception signal by using the estimation result.
The estimated transmission line includes influence of noise. As one of methods to suppress the noise, there is an art of carrying out averaging among adjacent samples in a frequency domain or a time domain. When carrying out averaging among the adjacent samples, each of desired signals that are held by the adjacent samples is in-phase each other, and consequently electric power of the desired signal does not decrease. On the other hand, a phase of the noise is random, and consequently electric power of the noise decreases in inverse proportion to number of the adjacent samples to which averaging is carried out. By using the above-mentioned characteristics, it is possible to decrease a ratio of the electric power of the noise to the electric power of the desired signal by increasing the number of the adjacent samples.
Here, when receiving a multi-path signal including a long delay time, a transmission line estimation result in the frequency domain includes influence of a phase fluctuation or an amplitude fluctuation among samples due to influence of a delay path. In the case of a general method of carrying out averaging to the transmission line in the frequency domain, when there is the phase fluctuation among the samples which are within an averaging range, averaging is carried out among the samples which lose the in-phase characteristics each other. Therefore, the general method of carrying out averaging to the transmission line in the frequency domain has a problem that an amplitude of the sample becomes small after carrying out averaging, and consequently accuracy in estimating the transmission line becomes degraded.
A patent literature 1 (hereinafter, denoted as PTL 1: Japanese Patent Application Laid-Open Publication No. 2010-246024) discloses an art to suppress the noise also in the case of a transmission line, which includes a path having a long delay time, by making paths, which are included in a delay profile, individually pass through infinite impulse response filters.
PTL 2 (Japanese Patent Application Laid-Open Publication No. 2011-217424) discloses an OFDM (Orthogonal Frequency Division Multiplexing) reception device which regenerate a data symbol by using a channel estimation value estimated from a pilot symbol. According to the device disclosed by PTL 2, since it is possible to carry out extrapolation in an effective out-of-band of the frequency domain in consideration of all path components in the time domain, it is possible to form a virtual waveform which sufficiently reflects various frequency components existing within an effective band.
According to the art disclosed by PTL 1, also when a delay time of an interference wave is long, it is possible to improve noise immunity. Here, in the case of the art disclosed by PTL 1, there is a possibility that, since a filter coefficient is selected out of a plurality of filter coefficients which are set in advance, and a filter having the selected filter coefficient is used, a path may exist at an edge of a pass-band of the filter. Therefore, the art disclosed by PTL 1 has a problem that, when the path exists at the edge of the pass-band of the filter, a side-robe of the path is removed, and consequently accuracy in estimating the transmission line becomes decreased.
According to the device disclosed by PTL 2, it is possible to find out the channel estimation value with accurately suppressing the noise also in the case of the transmission line which has large delay variance in the time domain. Here, according to the device disclosed by PTL 2, the noise is suppressed by replacing a complex amplitude value of an index in a band other than a path position of the delay profile with 0. Therefore, the device disclosed by PTL 2 has a problem that it is necessary to estimate the channel estimation value in the band other than the path position of the delay profile.